Cube won't appear in render
Possible causes for a Blank render:

No lights in scene.

Objects disabled for Render in outliner (camera icon).

Strips in VSE and Sequencer enabled.

Objects in different layer, or in a layer disabled in Render Layers.

Object blocked by other objects.

Active camera aiming in a different direction.

An object in the scene set as active camera.

Any, some or all of the above.